This invention relates to an improved method of controlling the rate of a fluid flow by means of an electromagnetic flow control valve of the on-off functioning type.
It is a well known technique to control the rate of flow of a fluid, either gas or liquid, by using an electromagnetic flow control valve of the on-off functioning type and by operating this valve with a pulse signal of which duty, that is, the proportion of pulse durations to pulse intervals during a definite amount of time, is varied thereby varying the flow rate proportionally to the duty of the pulse signal. Usually the pulse signal is of a constant frequency so that the duty is varied by pulse duration modulation. In practical applications, an advantage of this technique over the use of an electromagnetic flow control valve of the proportional type such as an electric-hydraulic servo valve, which exhibits a strictly linear control characteristic in accordance with the level of a control signal, is the possibility of providing relatively inexpensive flow control systems. In the automobile industries, for example, this technique has been employed to control the flow rate of either auxiliary air introduced to regulate the air-to-fuel ratio or exhaust gas recirculated to reduce the emission of nitrogen oxides.
In conventional flow rate control methods utilizing an electromagnetic flow control valve of the on-off functioning type, it is usual to render the flow rate maximum or minimum by maintaining the duty of the control pulse signal at 100% or at 0%, meaning that temporarily the pulse signal takes the form of a continuous signal of a constant level, thereby keeping the electromagnetic valve in the ON-state or in the OFF-state.
As a problem in the practice of such control methods, under the 100% duty and 0% duty conditions it is difficult to ascertain properness of the supply of the pulse signal and/or the function of the electromagnetic valve. For example, there is the possibility of misunderstanding that the duty of the control pulse signal is kept at 0% while in reality the supply of the intended control pulse signal is interrupted by failure of the control circuit. It may be wished to check the function of the electromagnetic valve by detecting changes in the voltage at the input terminals of the electromagnetic valve (the changes corresponding to the frequency of the pulse signal) with an ordinary instrument such as a counter. While the duty of the control pulse signal is either 100% or 0%, however, there occurs no change in the voltage at the terminals so that the counter continues to give a reading representative of a state just before the shift of the duty to 100% or 0%. Hence, it is impossible to discriminate between the 100% or 0% duty condition and the failure of the signal generating circuit.